The demand for safe and effective therapeutic technology continues to increase as worldwide cancer diagnoses increase. High intensity focused ultrasound (HIFU) is a promising option to focus acoustic energy for non-surgical ablation of tumors, while avoiding surgical complications and long recovery times. With HIFU, acoustic energy is delivered to a target location, and in the process, causes warranted damage by over-heating the tissue and/or causing mechanical injury. There remain concerns with HIFU regarding long surgical time, high energy requirements, and off-target effects, such as skin burns, due to acoustic energy delivery requirements. A new technology which could increase the amount of heating delivered to the site of pathology, while reducing off-target thermal damage would be highly clinically significant. Microbubbles have shown potential to decrease energy requirements by providing enhanced conversion of acoustic energy to thermal and mechanical energy through cavitation. Limitations in microbubble diameter prevent extravasation outside the circulatory system, and their thin shells give them a very short half-lif in vivo. Phase-changing nanoagents (PCNAs), a novel nanoparticle in the field of ultrasound, can be developed by compressing the gas-core of microbubbles into a liquid. These agents possess a diameter range of several hundred nanometers, likely capable of extravasating through the leaky vasculature of tumors and accumulating in tumors due to the enhanced permeability and retention (EPR) effect. In the following proposal we aim to optimize nanoagent formulation and improve HIFU performance through further development of PCNAs. Little has been done in regards to evaluating and optimizing in vivo PCNA behavior, specifically optimizing their extravasation and accumulation potential as well as evaluating their circulation time in vivo. We propose to work towards generation of a PCNA designed to accumulate in tumors and provide sustained tumor circulation, which can be used for HIFU ablation effectively. PCNA properties will be modified by varying the type of perfluorocarbon gas used, the diameter distribution of PCNA formulation, and the shell composition. Ideal nanoagents for HIFU will have properties of long circulation time, long accumulation time inside tumors with little deposition in healthy tissue, and enhanced ablation at the target with minimal off-target damage. PCNA formulations will be tested in the treatment of mouse melanoma tumors to test this hypothesis. It is the hope that the successful completion of all three proposed aims will demonstrate the versatility of the nanoagents and advance the PCNA technology for clinical applications in ultrasound for therapy and diagnostic imaging.